1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates an image forming apparatus that irradiates a photosensitive member with light corresponding to image data, and forms a latent image on a photosensitive member.
2. Description of the Related Art
In an image forming apparatus, such as an electrophotographic copying machine or a laser beam printer, using a laser beam, scanning lines for a plurality of lines can be drawn with a plurality of laser light-emitting sources for high-speed and high-resolution printing, as discussed in Japanese Patent Application Laid-Open No. 03-200917.
However, since optical paths of respective laser light beams differ, positions where respective laser light beam pass through an optical system also differ, so that curves or inclinations different depending on lasers may occur on scanning lines. In this case, an interval (pitch) of the scanning lines drawn by lasers may vary depending on the main scanning position. If the pitch of the scanning lines is not uniform, unevenness in density may occur on a screen image which is uniform in density inside one page, at the background, or in the predetermined region.
This issue will be described in detail. FIG. 13A illustrates scanning lines caused by respective lasers when scanning lines for four lines are simultaneously drawn by four laser light-emitting sources. The solid line indicates an ideal scanning line. The dashed-dotted line indicates a scanning line by laser 1. The dashed-two dotted line indicates a scanning line by laser 2. The dotted line indicates a scanning line by laser 3. The broken line indicates a scanning line by laser 4. In FIG. 13A, the scanning lines by laser 1 and laser 2 are curved convex upward, the scanning line by laser 3 is not curved, and the scanning line by laser 4 is curved convex downward. As can be seen from FIG. 13A, the pitch of the ideal scanning lines is fixed irrespective of the main scanning position. On the other hand, the pitch of the actual scanning lines in the sub scanning direction differs depending on the main scanning position.
FIG. 13B is an enlarged view of scanning lines in a main scanning position A in FIG. 13A. In the main scanning position A, which is the left end of main scanning, the actual scanning line is present on an ideal scanning line. The scanning lines (solid lines) are drawn at an ideal pitch α. FIG. 13C is an enlarged view of scanning lines in a main scanning position B in FIG. 13A. In the main scanning position B, which is in the vicinity of the center of main scanning, the actual scanning lines (dashed-dotted lines, dashed-two dotted lines, dotted lines, and broken lines) deviate from the ideal scanning lines. The actual scanning lines deviate by distances β1, β2, β3, and β4, respectively, from the ideal scanning lines.
A case where a screen image is formed by such scanning lines that are non-uniform in pitch in the sub scanning direction will be described. FIG. 14A illustrates a halftone dot image obtained by expressing a multilevel halftone image with binary values by screen processing. This screen processing is executed by using a dither filter of 4 by 4 pixels. Among 4 by 4 pixels of the dither filter, central 2 by 2 pixels are portions where a laser spot is formed by laser light. In FIGS. 14B to 14D, the solid line indicates an ideal scanning line, the dashed-dotted line indicates a scanning line drawn by laser 1, the dashed-two dotted line indicates a scanning line drawn by laser 2, the dotted line indicates a scanning line drawn by laser 3, and the broken line indicates a scanning line drawn by laser 4. FIG. 14B illustrates laser spots for every halftone dot at the end part of main scanning formed during scanning by laser 1 and laser 2 (or laser 2 and laser 3, laser 3 and laser 4, or laser 4 and laser 1). At the end part of main scanning, all lasers can execute scanning at an ideal pitch. Thus, a halftone dot can be formed.
FIG. 14C illustrates laser spots for every halftone dot in the vicinity of the center of main scanning formed during scanning by laser 4 and laser 1. In the vicinity of the center of main scanning, since the scanning line by laser 4 is curved convex downward, a laser spot is formed below the ideal scanning position. Since the scanning line by laser 1 is curved convex upward, a laser spot is formed above the ideal scanning position. Accordingly, compared with a case in which the scanning line is formed on the ideal scanning line, the halftone dot is reduced in the sub scanning direction. Since the number of laser light-emitting sources is four and the vertical size of the dither filter is four pixels, in this case, central 2 by 2 pixels among 4 by 4 pixels of the dither filter are formed by the same laser light-emitting sources (e.g., laser 4 and laser 1). Accordingly, the size of a halftone dot at the center part of main scanning becomes smaller than that at the end part of main scanning throughout the area in the sub scanning direction. Thus, as illustrated in FIG. 15A, the density is expected to be uniform at the entire area. However, as illustrated in FIG. 15B, the density at the center part of main scanning becomes thinner than that at the end part of main scanning.
On the other hand, as illustrated in FIG. 14D, when a half tone dot is formed by laser 2 and laser 3, in the vicinity of the center of main scanning, the scanning line by laser 2 is curved convex upward and the scanning line by laser 3 is not curved. Thus, compared with that when the scanning line is formed on the ideal scanning line, a halftone dot is enlarged in the sub scanning direction. Accordingly, the size of a halftone dot at the center part of main scanning becomes larger than that at the end part of main scanning throughout the area in the sub scanning direction. Thus, as illustrated in FIG. 15A, the density is expected to uniform at the entire area. However, as illustrated in FIG. 15C, the density at the center part of main scanning becomes thicker than that at the end part of main scanning.